Turret mooring assemblies are known on hydrocarbon production and/or processing vessels and comprise a mooring buoy and a turret mooring structure. The mooring buoy is anchored to the seabed with anchoring legs. The turret mooring structure, provided on a vessel, has a cavity for receiving the mooring buoy and one or more buoy locking devices for locking the mooring buoy in the cavity.
The turret mooring structure may be an internal turret mooring structure or an external turret mooring structure. An internal turret mooring structure is provided within the hull of the vessel, in a so-called moon pool. The cavity is formed as an opening at or near the bottom of the vessel, facing downwards. An external turret mooring structure is provided outside the hull of the vessel. The external turret is fixed with suitable connection members at an outboard position at the bow or stern of the vessel.
The mooring buoy may be moved up and down, i.e. from a storage position at a safe distance below the water surface (e.g. 30-200 meters) to a mooring position close to or at the surface of the water where it can be received by the cavity.
The turret mooring structure itself is connected to the vessel, but is rotatable with respect to the vessel, allowing the vessel to weathervane under influence of wind, waves, currents and drifting ice. The turret mooring system may be disconnected and reconnected when needed, thereby providing a disconnectable turret mooring system.
The turret mooring system comprises a fluid transfer system to allow transportation of hydrocarbons, such as oil or gas, for instance by establishing a flow path between the vessel and a subsea well via the turret mooring system and the mooring buoy.
The turret mooring structure may comprise a first part of the fluid transfer system and the mooring buoy may comprise a second part of the fluid transfer system. The turret mooring structure may comprise a turret manifold and the mooring buoy may comprise a buoy manifold, both manifolds each comprising at least one conduit. The turret and buoy manifold are matching such that conduits of the turret manifold can be connected to corresponding conduits of the buoy manifold of the fluid transfer system to establish a flow path.
During the connection of the mooring buoy to the turret mooring structure, the mooring buoy is locked in a fixed position with respect to the turret mooring structure inside the cavity.
According to state of the art in turret mooring systems, centering of the mooring boy in relation to the turret is carried out while locking the mooring buoy into the cavity of the turret mooring structure.
International patent applications WO1993011030-WO1993011035 disclose locking mechanisms of a turret mooring structure comprising a plurality of locking fingers distributed around an annular locking shoulder of the buoy, for releasable locking of the outer member of the mooring buoy in the receiving space. The mechanism comprises a pair of locking dogs which are actuated by a hydraulic system and are rotatable about horizontal axes at diametrically opposite sides of the receiving space. If desired, more than two locking dogs may be provided. The hydraulic actuators for operation of the locking dogs may for example be hydraulic cylinders. When activating the locking dogs these will pivot in a vertical plane to engagement with the downwards facing abutment edge of the upper cone member.
International patent application WO2001089919 discloses a mechanism for releasably locking of an element in relation to a base, especially for the locking of a buoy in a downwardly open receiving space in a floating vessel. The mechanism comprises a rotatably mounted locking arm which is pivotable between a release position and a locking position in which an abutment edge on the locking arm is in engagement with an abutment edge on the element to be locked, a linkage which is connected between the locking arm and the base and which, in the locking position, is in a self-locking over-centre position, and a driving means for actuation of the linkage. The linkage comprises a length-adjustable first link which, from an initial position with the locking arm close to its locking position, is arranged to be extended to thereby pivot the locking arm additionally to a final locking position, for achieving a desired preloading force in the engagement between the abutment edges of the locking arm and the element. The first link preferably is a hydraulic cylinder having a piston rod of which one end is connected to the locking arm.
From WO 2010/081826 a turret mooring assembly according to the preamble of claim 1 is known. The upper rim portion of the known riser supporting buoy is engaged by a number of hydraulically actuated locking members, that exert an upward force on the buoy. The upper rim portion of the known buoy is clamped in an upward direction against an abutment ring of the receiving cavity. The upper rim portion of the buoy is very stiff and loads on the buoy are al transferred into the turret via the locking members. Cyclic loading of the locking members can lead to reduced fatigue life and malfunctioning of the locking members. Furthermore, in the known locking structure the deflections between the upper part of the buoy and the receiving cavity may be relatively large, resulting in difficulties in applying a water-tight seal between the top of the buoy and the cavity wall.
It is therefore an object of the present invention to provide a mooring system that overcomes one or more of the disadvantages from the prior art. It is in particular an object of the invention to provide a mooring system in which the loads on the locking device are reduced.
It is again an object of the invention to provide a mooring system that allows effectively applying a water-tight seal between the buoy and the cavity wall.